Nuclear reactions resumed at the Chernobyl nuclear power plant: can the catastrophe be repeated

At the Chernobyl nuclear power plant, neutron activity was recorded. Could this provoke another disaster? Фокус.

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To date, researchers have argued that current levels will not trigger a fission chain reaction.

“It's like embers on a barbecue,” says Neil Hyatt, a nuclear chemist at the University of Sheffield. Ukrainian scientists are trying to determine if the reactions will go away on their own or if extraordinary measures will be required to prevent another accident.

“The sensors track a growing number of neutrons, a fission signal emanating from one inaccessible room,” says Anatoly Doroshenko of the Institute for Nuclear Power Plant Safety.

“There are many uncertainties,” says Maxim Savelyev of the Institute for Nuclear Power Plant Safety. "But we cannot rule out the possibility of an accident."

Although the number of neutrons is increasing, the management of the power plant has several years to figure out how to prevent a catastrophe.

“Any decision taken by management to resolve the situation will be of great interest to Japan, which is trying to cope with the consequences of its own disaster 10 years ago in Fukushima,” Hyatt says.

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When an accident occurred at the Chernobyl NPP on April 26, 1986, part of the fourth power unit melted, and the uranium rods, their zirconium shell, as well as graphite control rods and sand fell on the core of the power unit (trying to extinguish the fire). As a result, they merged into lava. The lava penetrated into the basement of the reactor hall and hardened there. This formation is called fuel-containing materials, which contain about 170 tons of enriched uranium - 95% of the original fuel.

A year after the accident, a sarcophagus called "Shelter" was built over the power unit. It allowed rainwater to penetrate inside, as it slows down or restrains neutrons, thereby increasing the likelihood of fission of uranium nuclei. But a large amount of water often causes a sharp increase in the number of neutrons.

The leadership of Chernobyl assumed that any risk of a critical situation would disappear when (in November 2016) a massive Arka object, a special insulating arched structure, was installed over the Shelter.

The “Arch” structure, worth $1,8 billion, was intended to isolate the “Shelter” (it was planned to be dismantled in the future). The “Arch,” among other things, protected the power unit from rain, and therefore the number of neutrons stabilized and even began to decrease.

But in some places they started to grow and almost doubled in 4 years in room 305/2, where tons of fuel-containing materials are buried under rubble.

A study by the Institute for the Safety of Nuclear Power Plants suggests that drying up the fuel somehow makes the neutrons passing through it more, not less, at fissioning uranium nuclei.

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“This is very plausible data,” agrees Hyatt. "It's just not clear what the mechanism might be."

As the water continues to recede, there is concern, the nuclear chemist suggests, that "the fission reaction is accelerating exponentially," leading to "an uncontrolled release of nuclear energy."

A runaway fission reaction in fuel-containing materials can occur after the heat from the fission has evaporated the remaining water. Nonetheless, Savelyev notes, while any explosive reaction will be contained, it could bring down unstable parts of the shaky Shelter and fill the Arka with radioactive dust.

Eliminating this threat is a difficult task. The radiation level makes it difficult to get close to the building to install sensors. In addition, it is impossible to spray gadolinium nitrate on the nuclear debris here, since it is under concrete.

One idea is to create a robot that can withstand the intense radiation long enough to drill holes in fuel-containing materials and insert boron cylinders that will function as control rods and absorb neutrons. In the meantime, the Institute for the Safety of Nuclear Power Plants intends to strengthen monitoring of two other areas where the level of fuel-containing materials (FCM) may become critical.

Renewed nuclear fission reactions are not the only problem facing Chernobyl leaders. Under the influence of intense radiation and high humidity, the FCM disintegrate, generating even more radioactive dust, which complicates the plans to dismantle the Shelter. In the beginning, the formation of the SCM called "Elephant's Foot" was very hard, which forced scientists to use a Kalashnikov assault rifle to cut a piece for analysis. “Now it has the consistency of sand,” says Savelyev.

Ukraine has long wanted to move FCM and store them in geological storage. By September, with the help of the European Bank for Reconstruction and Development, it is planned to develop a special plan for this. But since "life" at Shelter is still "seething", it may be harder than ever to bury the remains of the reactor.

In response to the information that appeared on the site of the Chernobyl nuclear power plant a denial was made public.

“After the installation of the“ Arch ”, some sensors record an increase in neutron activity in the accumulations of fuel-containing materials in the melting zone of the sub-reactor plate in room 305/2. Such a situation was predicted even before the construction of the "Arka" and is due to the process of water loss by an excessively humid environment of the layer of black lava-like FCM (LFCM), which are located above the potentially nuclear hazardous accumulations (NOC) of FCM. The AEC experts predict that in the future the influence of the FCM NOS on the level of neutron activity, which is registered by the sensors, will become constant. The total neutron activity will continue to increase slightly due to a decrease in the water level in the LFCM layer, which is located above the NOC of the FCM, until the optimal ratio of nuclear fuel and water is reached. During further dehydration of this medium, a slight drop or an abnormally long period of unchanged parameters of the neutron flux density (PNF) is predicted, ”the message says.

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To ensure the guaranteed safety of the Shelter object, constant monitoring of the FCM accumulation inside the facility is carried out, which is monitored by the Nuclear Safety Monitoring System (NSC), which is part of the Shelter's integrated automated control system.

“Over the entire observation period since the beginning of the establishment of the“ Arka ”, no excess of the limits of safe operation was observed. At present, the readings of the sensors in all rooms have stable values ​​without growth tendencies, the current levels do not pose a threat of the emergence of a self-sustaining chain reaction of fission, ”the ChNPP management notes.

To ensure the safety of the Shelter, there are systems that perform safety functions, including systems for maintaining the FCM in a subcritical state. In case of exceeding the limits of safe operation according to the readings of the sensors, a neutron-absorbing solution is introduced into the growth zone (in accordance with the technological regulations of the Shelter object).

In order to assess the current state of FCM and predict their long-term behavior, the Institute for NPP Safety Problems in 2018 developed a Program for monitoring fuel-containing materials (FCM) of the OS (nuclear and radiation safety of the OS).

The results that will be obtained after the completion of work under this program are used for the development (if necessary) and implementation of organizational and technical measures to prevent risks from the adverse consequences of deterioration of the FCM condition. But the monitoring of the FCM state, the publication says, must be carried out for several years, when all processes of changes in the temperature and humidity regime in the Shelter object are stabilized and it will be possible to more reliably predict the FCM behavior, as well as guarantee their support in a nuclear safe state.

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